Energy (motion, heat, light, sound)

The idea

What do a rolling ball, a glowing lamp, a beating drum, and a warm sunny patch on the floor have in common? Each one shows energy at work — energy is what makes things happen. It comes in kinds you can spot with your senses: motion energy in things that move, heat energy in things that are warm, light energy in things that glow, and sound energy in things you can hear. Wherever something is going on, energy is there making it go.

Energy can also be stored up and saved for later, then let out. A stretched rubber band, a wound-up toy, and the food in your lunch all hold stored energy, ready to turn into motion, sound, or warmth. A common mix-up is thinking energy only means electricity from a plug or a battery. Electricity is just one carrier. Your own body turns breakfast into running and shouting at recess — that is energy changing from one kind to another, with no wires anywhere.

Worked example

You have a wind-up walking robot toy. You wind the key 5 turns, let it go, and it walks and clicks for about 4 seconds. Then you wind the key 10 turns and let it go. What do you expect, and what does the toy show about energy?

1. Think about where the toy's go comes from. Your hand turns the key, which tightens a spring inside — winding stores energy in the spring.
2. Watch the 5-turn test: the robot walks and clicks for about 4 seconds, then stops. The stored energy came out as motion energy and sound energy until the spring ran down.
3. Predict the 10-turn test before letting go: twice the winding should store about twice the energy, so the robot should keep going for roughly twice as long.
4. Let it go and count: the robot walks for about 8 seconds this time. More stored energy meant more motion and more sound, just as predicted.
5. Make sense of it: the walking did not come from nowhere — every second of it was paid for by the winding your hand did first.

Answer. The 10-turn robot walks for about 8 seconds, roughly twice as long, because more winding stores more energy, which comes back out as motion and sound.

Check your understanding

• What kinds of energy can you spot in your kitchen at breakfast time, and what is each one doing?
• Where does the energy that moves your legs at recess come from, if you trace it back step by step?
• How is a stretched rubber band like a wound-up toy, and what happens to its stored energy when you let go?
• Why does the robot toy always stop walking after a while, even though nothing visible touches it?

Keep going

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